It is a way of adding disk space without changing any of the logical pointers. Normally when you add a disk it would have a drive letter assignment. It's like a 55gal drum connected at the top to another 55gal drum. When the first fills up it spills into the second. If that pointer between the drums gets hosed you lost everything. I see it as a way to overcome poor planning or for storing noncritical but backed up data. If you were creating huge temp files it would be useful for that. Using it for anything else is foolhardy.
Othehill you can't make a spanned volume fault tolerant. You can't mirror it using dynamic disks. When you see statements like "spanned disks can be mirrored" they are talking Raid 0+1 which you can not do with dynamic disks.
I have seen all sorts of 'creative' configs. Hardware raid5 with 3 disks then they add a disk and span the data volume. Boy are they in for a surprise! Lose the spanned disk you lost the data volume. But its raid5 they cry! But you created a non fault tolerant disk on a fault tolerant array. You build a sand castle on bedrock it will still wash away.
Definations have changed in the 17 plus years I have been in the industry. Jbod was nothing but a collections of disk off a raid controller with no raid config. For some now its a spanned volume. Kinda like MS changing boot and system disk definations from dos days.
Are you ready for where Microsoft wants you to go today?